for instruments observing in the mid-IR (CRIRES and VISIR), the allowable maximum Precipitable Water Vapour (PWV)

the allowable twilight constraint that defines the earliest time in minutes with respect to the end of the astronomical twilight when the execution of the OB can be started

the allowable absolute time window (i.e. for time critical events, multi-epoch monitoring)

the allowable local sidereal time range (e.g. for ADI observation)

for VLTI instruments, the availability of the desired baseline

The Observing Constraints are specified by the user at Phase 2 for each Observation Block. Since the execution conditions required by each programme are an important ingredient in the process of building up the Long Term Schedule of an observing semester, and thus determines which programmes can or cannot be scheduled, users are not allowed to specify at Phase 2 constraints that are more strict than those specified in the original proposal. Users can however relax the constraints during the submission of their Phase 2 material. The values in the OB constraint sets that are selected (and approved) during Phase 2 preparation (and review) cannot be changed later during the observing period.

General Classification Rules

Quality Control of OBs executed in Service Mode will be based on the user's specified constraints for airmass, atmospheric transparency, seeing (i.e. image quality), moon constraints, as well as Strehl ratio for Adaptive Optics mode observations.

Note: the seeing constraint as defined in the OB is judged against the full width at half maximum (FWHM) of a point source in the resulting image (or spectral image), i.e. at the observed wavelength, for most of the VLT instruments (i.e. it is the image quality).

Additional Observing Constraints and Classification Rules for HAWK-I

Lunar constraints

Do not overspecify the Moon constraints as this reduces the chances of the Observing Block being executed! The Moon does not directly affect infrared observations. For most HAWK-I observations the Moon illumination (FLI) can be entirely relaxed by selecting FLI=1. Only for the shortest wavelength filters (<1.2mic) there is an increased contribution from the sky background, degrading the sensitivity of your observations.

Twilight constraints

Because HAWK-I is an infrared imager, observations of bright objects may be carried out in twilight. From P91 onwards, there is a new constraint in P2PP3 called twilight constraint. This constraint can be used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small. Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight. The twilight constraint can take values between -45 and 0 minutes.